The full phrase is "Stop! Hammertime!" but I'm a lousy conformist, so I put it here. It is by far the most awesome phrase ever. You could make Slimer from Ghostbusters say it and it would still make sense. Its inventor was nothing short of genius as well.

'Hammertime' is a brilliant surprise attack on a romantic lovemaking session. Whilst you and your partner are engaged in some slow, passionate, romantic sex, look into her eyes, wait for her to move her head forwards to kiss you, then shout, "STOP, IT'S HAMMERTIME!" and proceed to bang her like a drum at speed while shouting, "WHOOA-O, WHOOA-O! WHOOA-O, WHOOA-O!"